1. Field of the Invention
The present invention relates to a wireless communication system for use with aircraft. More particularly, the invention relates to a method and system for providing wireless communication service in an air-space communication system.
2. Background Technology
The growth of telecommunications and the desire for mobile connectivity has increased the demand for public telecommunication services while in flight. In an effort to satisfy the demand for in-flight telecommunications, commercial aircraft are often equipped with a special ground-to-air communication system in order to provide passengers with public phone and/or data service. However, in-flight public phone systems largely use air interface protocols that are not compatible with conventional mobile telecommunication systems. Furthermore, airline passengers are typically not permitted to use personal mobile telephones while in flight due to possible interference with aircraft systems and with the ground-based wireless systems.
In contrast to in-flight phone and/or data systems, conventional mobile telecommunications systems permit mobile subscribers to access the PSTN (Public System Telephone Network) or other network by using a cellular infrastructure system intended for communications on the earth""s surface. Generally, mobile systems may be designed to maximize two equally important parameters. The first major parameter is coverage. Coverage provides an adequate radio communication link between the base station and the mobile user. The second major parameter is capacity. Capacity ensures that the system can handle the demand or load by mobile users collectively.
The mobile coverage area of a system is generally based primarily on the type of terrain, the transceiver equipment performance requirements for a given telecommunications standard, and the level of RF interference. Coverage is primarily limited by two major factors: the strength of the signal and the quality of the signal. The strength of the signal is based on the propagation characteristics of the signal. The quality of the signal is based on impairments to a signal, primarily due to interference.
The propagation characteristics of radio waves in free-space are different from the propagation characteristics of radio waves along the earth""s surface. Normally, radio waves propagate further in free space than along the surface of the earth. The free-space propagation of radio waves is characterized by the xe2x80x9cFrenel zone,xe2x80x9d which sets forth the spatial terrain requirements that will permit free-space propagation. As a result, communications between a base station and an aircraft may take place over greater distances than communications between a base station and a mobile unit located on the earth""s surface.
Presently, if an aircraft in an air-to-ground system transmits on the same frequencies as a ground cellular system, the air-to-ground system will likely interfere with the ground cellular system. Due to the extensive free-space propagation, transmissions from an airplane are broadcast over a large area of the earth""s surface, possibly interfering with one or more ground base stations. This is a primary reason why mobile subscribers are currently prohibited from operating mobile units, such as their personal telephones or other mobile communication devices, on aircraft.
For example, the propagation characteristics of radio waves in a typical suburban area at 800-900 MHz may be expressed as follows:
Received signal=xe2x88x9261.7 dBmxe2x88x9238.4 dB log radius (miles)
where the radius is the distance between the base station and the mobile station in miles, and the received signal is the signal strength in dBm at the receiver of the mobile station. William C. Y. Lee, xe2x80x9cMobile Communications Engineeringxe2x80x9d, McGraw-Hill, N.Y. 1982, pp 108. If the minimum receive signal at the receiver is xe2x88x92100 dBm, then the radius of a typical suburban cell site at 800-900 MHz is about 10 miles (16 Km). A ground base station cell radius may typically vary from approximately 0.5 kilometers to approximately 20 kilometers or more.
However, due to the extent of free space propagation, a similar base station adapted for in-flight communications may have a radius of up to 500 kilometers or more. The actual cell radius will depend on factors such as the power budget of the system, which takes into account transmit output power, path loss between the transmitter and the receiver, transmission line losses in, the transmit and receive paths, fade margin, and receiver sensitivity.
System capacity, the second parameter to be maximized in a mobile system, is a function of the available spectral bandwidth and the RF interference rejection performance of the system equipment. Mobile telecommunications systems utilize large amounts of bandwidth to provide commercial grade telephony service to tens or thousands of mobile users in a service area. Lower interference levels, and thus greater system capacity, results from having a larger spectral bandwidth. The FCC (Federal Communications Commission), however, limits spectral bandwidth allocations. Consequently, in order to meet subscriber service demands, RF engineering techniques such as interference analysis and optimization (e.g., frequency re-use, and power control) may be used to optimize this limited available bandwidth. Some of these techniques, however, adversely affect interference levels. Frequency re-use in many systems for instance, is a principle generator of RF interference in a mobile system.
Because system interference directly affects both capacity and coverage, characterizing such interference is desirable. Since most mobile systems are bi-directional interference may affect both a forward communication link and a reverse communication link. Communication from a base station to a mobile unit is known as the forward link. Communication from the mobile unit to the base station is known as the reverse link. Interference on the forward link exists when the mobile unit""s receiver experiences interference from base stations other than the intended base station. Interference on the reverse link exists when the transmit carrier from the mobile unit to the base station is interfered with by other transmitting mobile units in the system. In both the forward and the reverse links, the intended transmit carrier may be referred to as carrier xe2x80x9cCxe2x80x9d, and interference created by other mobile unit transmitters may be referred to as interference xe2x80x9cIxe2x80x9d.
The mobile subscriber units and system infrastructure communication equipment operating in a mobile system are typically capable of tolerating a specified maximum ratio of RF carrier to interference known as a C/I (Carrier-to-Interference) ratio. The C/I ratio may, for example, be based on an applicable telecommunications standard (e.g., IS-95, IS-54, G3, etc.). So long as the ratio of the intended receive carrier signal to the system interference at the receiver meets or exceeds the specified C/I ratio, communications between the mobile user and the base station are able to function properly. However, if the ratio of the intended receive carrier signal to the system interference at the receiver does not meet or exceed the specified C/I ratio, then the mobile unit""s receiver may not be able to reliably receive the signal, and communication may fail or be of poor quality. For example, a C/I ratio of 6 dB may be required for a CDMA (Code Division Multiple Access) air interface such as IS-95, while a C/I ratio of 18 dB may be required for an analog air interface standard such as AMPS (Advanced Mobile Phone Service).
Besides available spectral bandwidth, system interference is dependent upon the distribution of subscribers. For example, the maximum concentration of subscribers is significant, because, if subscriber concentration is high enough, then the C/I ratio may exceed a maximum allowable C/I ratio. If subscriber concentration is too high, then the air system may need to be reconfigured to reduce the interference.
One technique for controlling interference is to use a directional antenna to direct transmissions to a desired area. For example, a typical omni-directional cellular base station has a tower-mounted antenna that radiates outward radially from the antenna in a 360 degree pattern in the horizontal plane, but with a slight downward tilt to prevent radiating energy into the surrounding airspace. Similarly, a directional antenna transmits in less than a 360 degree pattern in the horizontal plane, and may also transmit with a slight downward tilt to prevent radiating energy into the airspace.
Radiation patterns along the earth""s surface are optimal for ground-based users because the radiated RF signal is directed approximately to the surface of the coverage area where antennas on mobile units are most likely to be located. Transmission into the airspace above the ground-based coverage area is undesirable in a conventional ground system because of the possibility of a direct or reflected wave causing interference elsewhere in the system. Therefore, conventional ground-based cellular telecommunications systems do not support air-to-ground communications, because the antenna radiation patterns avoid vertical radiation.
U.S. Pat. No. 5,878,345, (the ""345 patent) entitled xe2x80x9cAntenna For Nonterrestrial Mobile Telecommunication System,xe2x80x9d the contents of which are incorporated herein by reference, describes an antenna pattern that is substantially parabolic in shape, as is shown in FIG. 1. A disadvantage of such a parabolic antenna pattern is that an aircraft may experience gaps in coverage between cell sites because the horizontal radius of the cell site is smaller at lower altitudes. Therefore, an aircraft flying at a lower altitude may experience xe2x80x9cdead zonesxe2x80x9d in between cell sites, causing a loss of signal between the aircraft and the cell-site. The ""345 patent also proposes a substantially cylindrical radiation pattern, as is shown in FIG. 2. An antenna having such a pattern is difficult or impossible to design and build, however. FIG. 3 illustrates an antenna pattern occupying a central hole in a toroid and extending upward in a substantially conical manner. The deep null in the center of this radiation pattern has very little or no signal, which may result in a loss of signal between the antenna and an aircraft. In addition, the curvature of the pattern near the null is a problem for aircraft flying in a horizontal plane because the curvature of the toroid does not correspond to the shape of a typical flight path for an aircraft.
A further disadvantage of the system described in the ""345 patent is that the specified air interface differs substantially from standard air interfaces used in public ground-based systems. Consequently, users are unable to use their own personal mobile phones on the aircraft unless a base station is placed on the aircraft.
U.S. Pat. No. 5,444,762, entitled xe2x80x9cMethod and Apparatus for reducing interference among cellular telephone signals,xe2x80x9d the contents of which are incorporated herein by reference, proposes a single-loop aircraft antenna producing a radiation pattern having a substantially conical null above and below a toroid in a vertical plane at the aircraft. No base station antenna pattern is set forth. A vertical plane may be a plane indicating different levels of altitude relative to the earth. With such an aircraft antenna, the aircraft may experience gaps in coverage while flying over a base station, due to the nulls in the pattern. A further disadvantage is that the described aircraft antenna radiates both above and below the aircraft. Therefore, a portion of the energy radiated is wasted because communication will typically occur below the aircraft.
Thus, it would be desirable to provide an air-to-ground communication system in which an air-to-ground communications are supported between a mobile wireless communication unit on an aircraft and a base station located at or near the ground.
It would also be desirable to provide an air-to-ground communication system in which compliance with mobile system interference specifications is promoted.
It would additionally be desirable to provide an air-to-ground communication system in which a region""s air-to-ground base station antennas have radiation patterns that conform to typical flight paths or corridors for the region""s airspace. An air corridor may, for example, be a common flight path taken by aircraft.
It would further be desirable to provide interference isolation between a ground-based mobile system and an air-to-ground mobile system.
It would also be desirable to provide wireless communication service to an aircraft""s passengers, based on a conventional ground telecommunications standard so that the passengers could use their personal mobile units both on the ground and on an aircraft above the ground.
The present invention provides an improved method and system for providing air-to-ground communication services. According to an exemplary embodiment of the invention, an air antenna system defines a dome shaped radiation pattern having a large radius of curvature in the center of the dome. Alternatively, the air antenna system is defined by a cardioid radiation pattern. This arrangement provides interference isolation between an air-based system and a ground system by directing the coverage of the air system above a horizontal plane.
Two or more antenna elements may be combined to provide coverage specifically for an air corridor. By radiating only in the air corridor, interference in an adjacent ground system is reduced. Advantageously, an exemplary embodiment of the invention may provide the required levels of interference isolation between the air-based and ground system by isolating the coverage of the air system above a horizontal plane to an upward vertical direction.
It is thus an object to provide an air-to-ground communication system for air space coverage using a vertical antenna overlay onto an existing ground system. In accordance with an exemplary embodiment, an air base station is co-located with a terrestrial base station. The air base station includes a plurality of transceivers, an antenna system, and the necessary interconnecting hardware such as splitters and combiners. The base station may be configured to provide the required capacity of an air cell by adding transceivers using splitters and combiners. Coverage of an air cell may be achieved through the use of a stretched dome antenna or a plurality of directional antennas oriented in the specific regions requiring coverage.
The aircraft may communicate with the air base station via an external antenna mounted on the exterior of the aircraft. Passengers using their mobile phones, may originate and receive calls from within the aircraft via repeater equipment in the aircraft coupled to the external antenna.
Another object is to provide wireless communication service to an aircraft through the use of a passenger""s mobile telephone based on a conventional ground telecommunications standard. Further, techniques such as frequency reuse, adaptive power control and P.N. (pseudorandom number) reuse planning may be used to optimize the system.